As an example of a rotation sensor, U.S. Pat. No. 7,046,000B1 (related to JP2006-145528A and referred to as reference 1 hereinafter) discloses a rotation sensor, which includes a detecting element detecting a rotation of a rotational member and accordingly outputting a detection signal and which outputs a pulse in response to the rotation of the rotational member on the basis of the detection signal outputted from the detecting element. The rotational member, of which rotation is to be detected by the rotation sensor, includes plural teeth at an outer circumferential portion thereof. The detecting element is structured with a Hall element, a magnetic resistance element, or the like. Such detecting element is disposed to face one of the teeth of the rotational member. When the rotational member rotates, each of the teeth of the rotational member sequentially moves relative to the detecting element. When each of the teeth moves relative to the detecting element, the detecting element outputs the detection signal in response to a rotational speed of the rotational member. The detection signal outputted from the detecting element is an analog signal, i.e., a sine waved signal. The rotation sensor further includes a pulse generating portion, which generates the pulse in response to the rotation of the rotational member on the basis of the detection signal, so that the rotation sensor is applicable to a digital circuit of a microcomputers, for example.
Such rotation sensor executes an initializing process, such as a gain adjustment and an offset adjustment, in an initializing period defined from a moment where a power supply voltage is inputted to a moment where a predetermined amplitude fluctuation is observed. Further, the rotation sensor is applicable to detect a rotation of a wheel of a vehicle, a rotation of a gear of a transmission, or the like. When the vehicle is in a stopped state, the wheel or the gear of the vehicle is in a non-rotating state. However, because a vehicle body vibrates in accordance with an engine vibration, a distance between the rotational member and the detecting element may periodically fluctuate. In the initializing period, the rotation sensor determines whether the periodic amplitude fluctuation is due to the vibration or due to the rotation of the rotational member, in addition to executing the initializing process.
Further, the rotation sensor may determine a rotational direction of the rotational member in the initializing period. As disclosed in U.S. Pat. No. 6,492,804B2 (related to JP2001-165951A and referred to as reference 2 hereinafter), when the rotational direction is determined, the rotation sensor outputs pulse signals having different waveforms in response to the determined rotational direction. The rotation sensor thus indicates the rotational direction of the rotational member. Because the rotational direction is determined not only when the power supply voltage is inputted but also when the rotational directions are switched, an adjustment period corresponding to the initializing period is set even when the rotational member rotates. Further, the vibration and the rotational direction of the rotational member are required to be detected even when the rotational member temporarily stops to rotate. Accordingly, the adjustment period corresponding to the initializing period is set also when the rotational member temporarily stops to rotate.
In such initializing period, the adjustment period and in a vibration determining period, the rotation sensor is set not to output the pulse signal, i.e., an output of the rotation sensor is masked. In recent years, the output of the rotation sensor is utilized to a control device for an ABS (anti-lock brake system), a control device for switching power sources of a hybrid vehicle, for example. Such control devices require to rapidly receive information regarding the rotation of the rotational member. Accordingly, even in a predetermined period such as the initializing period and the adjustment period where the rotation of the rotational member may not be detected, the rotation sensor is required to rapidly output such information regarding the rotation of the rotational member.
A need thus exists for a rotation sensor which is not susceptible to the drawback mentioned above.